468 RIESS ON THE INCANDESCENCE OF 
The retarding power of a wire is therefore solely dependent 
upon the manner in which the discharge progresses through it ; 
its mutability up to a certain point, and subsequent continual 
alteration until the wire is destroyed, shows therefore that in 
every wire weak discharges are propagated in a different manner 
than strong ones. I shall endeavour in the sequel to explain 
hypothetically the nature of this different manner of propagation, 
and as the propagation of more powerful discharges produces the 
mechanical and calorific effects spoken of in this treatise, I shall 
at the same time endeavour to assign a reason for these effects. 
The mechanism of incandescence may be thus expressed: The 
incandescence of a wire is produced by a propagation of the elec- 
trical discharge through it, in a manner quite distinct from that 
which causes the wire to become heated. 
ON THE DIFFERENT MopeEs OF PROPAGATION OF THE 
ELECTRICAL DISCHARGE. 
Different Modes of Propagation in Metallic Wires. 
Whatever view is taken of the nature of electricity and of 
electrical conduction, we are forced to assume that in the metals 
the electrical state is communicated successively from one mo- 
lecule to the next. If therefore a piece of metal is electrified at 
one part, it can only be brought back again to an unelectrified 
condition after every particle has become first electrified and 
then unelectrified. In some cases two points are discernible in 
every part of the piece of metal which chronologically assume 
these two states one after the other; the straight line uniting 
these two points is then called the line of propagation of the 
electricity. A battery discharged by a homogeneous cylindrical 
wire affords such an instance. The discharge is effected by the 
wire connecting the two coatings becoming in a series of succes- 
sive pulsations first electrified and then unelectrified, and in 
every part of the wire that end which is nearest to the inner 
coating of the battery must become by the electricity of that 
coating sooner electrified than the more distant end; all the 
parts however which lie in a normal section of the wire must be 
simultaneously electrified and simultaneously unelectrified. The 
discharge of an electrical battery must therefore be considered 
to take place by the progression of a certain electrical state from 
one section of the homogeneous connecting wire to the next im- 
